The present invention relates to molten metal level measuring apparatus for continuous casting machines and more particularly to a molten metal level measuring apparatus of the type which employs an eddy-current type distance measuring device for molten metal level measuring purposes.
It has been disclosed for example in U.S. Pat. No. 4,186,792 to measure the level of the molten metal in a mold and control the molten metal level at a constant level in the continuous casting of the metal. In this case, by measuring the level of the molten metal in the mold with greater accuracy and controlling the molten metal level at a constant level, it is possible to improve the quality of cast billets and therefore it is desirable to use a nighly accurate mold metal level measuring apparatus.
A known mold metal level measuring apparatus of this type employing an eddy-current type distance measuring device is disclosed for example in Japanese Patent Laid-Open No. 57-192805. This eddy-current type distance measuring device includes a sensor coil having two coils which are wound and arranged one upon another on a vertically arranged hollow bobbin and the coils are connected differentially. Thus, the sensor coil is provided with a detection sensitivity only in its axial direction and the two coils function to cancel the effect of any electrically conductive member such as the mold wall located near to the sensor coil and the effect of temperature changes thereby accomplishing a distance measurement with greater accuracy.
However, in the case of the continuous measurement of the liquid level in the mold of a continuous casting machine in operation by using the eddy-current distance measuring device of the differential coil type, there still remain the following problems to be solved. More specically, while there will be no problem if the two coils of the sensor of the eddy-current distance measuring device are equally subjected to the electromagnetic effect of the mold inner wall, the sensor is in fact positioned in the upper part of the mold and therefore the lower coil undergoes the effect of the mold inner wall more than the upper coil. This tendency is increased as the sensor is located closer to the mold inner wall with the result that if particularly the cross-sectional area of the mold is small, the measurement error due to the difference in effect of the mold inner wall between the upper and lower coil is no longer negligible.